Circuit and a method for configuring pad connections in an integrated device

ABSTRACT

An integrated device includes a configuration circuit that is coupled to first and second bond pads and first and second conductive paths of the integrated device. The circuit receives a map signal that has a first value during a first operational mode of the integrated device and a second value during a second operational mode of the integrated device. In response to the first value, the circuit couples the first pad to the second conductive path. In response to the second value, the circuit couples the first pad to the first conductive path and the second pad to the second conductive path. The first operational mode may be a wafer test mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/115,103, filed Jul. 13, 1998, now U.S. patent application Ser. No.6,133,053, which is a divisional of U.S. Pat. Application No.08/619,261, filed Mar. 18, 1996, which issued as U.S. Pat. No. 5,796,266Aug. 18, 1998.

TECHNICAL FIELD

The present invention relates generally to electronic devices, and morespecifically, to a circuit for dynamically configuring bond-padconnections for different operational modes of an integrated circuit.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, one or more dies 2 are formed in a conventionalmanner on a wafer 4, which is formed from a semiconductor material suchas silicon. The dies 2 are integrated circuits or devices that have beenformed, but have not been detached from the wafer 4. For clarity, onlyone row of dies 2 is shown, but will be understood that generallymultiple rows of dies 2 are formed to substantially fill the wafer 4.During a wafer test procedure, conventional apparatus (not shown)electrically tests the dies 2. The testing apparatus includes probesthat contact selected ones of the bond pads (not shown) of the dies 2.

A limitation associated with such a wafer test procedure is that eachbond pad that will receive a signal from the testing apparatus oftenmust be placed only along the sides 8 of the dies 2 in order to performsimultaneously testing of multiple dies 2. Because the dies 2 are placedrelatively close together along their sides 6 to maximize the area ofthe wafer 4 occupied by the dies 2, the bond pads that are located alongthe adjacent sides 6 are often inaccessible to the probes of the testingapparatus, particularly when all of the dies 2 on the wafer 4 are testedsimultaneously. That is, the probes of the testing apparatus can oftenonly contact the accessible bond pads that are located along the othersides 8 of the dies 2. (The dies 2 are typically formed in the wafer 4such that there is sufficient clearance for the test probes to accessthe sides 8 of each of the dies 2.) Requiring he bond pads that are usedduring the wafer test procedure to be located only along the sides 8 maycause inefficient and complex circuit layouts on and increase the areasof the dies 2.

Referring to FIG. 2, which shows a top view of a die 2 of FIG. 1, aknown solution to this limitation is discussed. For clarity, the wafer 4and the remaining dies 2 of FIG. 1 are omitted from FIG. 2. The die 2includes accessible test pads 10 and accessible bond pads 14, which arelocated along accessible sides 8, and inaccessible pads 12, which arelocated along inaccessible sides 6. For clarity, FIG. 2 shows only twotest pads 10 a and 10 b, two inaccessible bond pads 12 a and 12 b, andtwo accessible bond pads 14 a and 14 b, it being understood that the die2 may include more or less of each of these pads. Each test pad 10 iselectrically coupled to circuitry (not shown) that is coupled to acorresponding pad 12 and that is to receive a signal from the testingapparatus during a wafer test procedure. Thus, by physically accessingtest pads 10, the testing apparatus can electrically access thecircuitry that is coupled to the inaccessible pads 12. Once the test iscomplete, however, the pads 10 typically serve no further purpose.

A limitation of this known solution is that the length of the accessiblesides 8 must be sufficient to accommodate the required number of thepads 14 and the test pads 10. Thus, the test pads 10 often increase thelength of the sides 8, and thus often increase the area of the die 2.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, an integratedcircuit is provided. The integrated device includes a circuit that iscoupled to first and second bond pads and first and second conductivepaths of the integrated device. The circuit receives a map signal thathas a first value during a first operational mode of the integrateddevice and a second value during a second operational mode of theintegrated device. In response to the first value, the circuit couplesthe first pad to the second conductive path. In response to the secondvalue, the circuit couples the first pad to the first conductive pathand the second pad to the second conductive path.

An advantage provided by one aspect of the invention is a reduction inthe number of test pads required in a die.

An advantage provided by another aspect of the invention is a reductionin the area of a die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a semiconductor wafer having dies formedthereon as is known in the art.

FIG. 2 is a top plan view of a die of FIG. 1.

FIG. 3 is a top plan view of a die formed in accordance with the presentinvention.

FIG. 4 is a block diagram of one embodiment of the configuration circuitof FIG. 3.

FIG. 5 is a schematic diagram of one embodiment of the configurationcircuit of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a top plan view of a die 16 that is formed in accordance withthe present invention. For clarity, the wafer in which the die 16 isformed, and the other dies formed in the wafer, are omitted. The die 16includes sides 8, which are accessible to a testing apparatus (notshown), and sides 6, which are substantially inaccessible to the testingapparatus during certain testing procedures, such as simultaneoustesting of multiple dies 2. One or more bond pads 12 and 14 are locatedalong the sides 6 and 8, respectively. For clarity, FIG. 3 shows onlyone inaccessible pad 12 and one accessible pad 14. A conventionalcircuit B is coupled to the pad 12 while the circuitry of the die 16 isin a first or normal mode of operation. In order to test the circuit B,a test signal must be applied to the circuit B. However, the pad 12 thatis coupled to the circuit B is inaccessible and thus cannot be used toapply the test signal to the circuit B. A conventional circuit A iscoupled to the pad 14. The circuit A is of the type that either need notbe tested or need not be tested at the same time that the circuit B isbeing tested. Since the pad 14 is not needed by the circuit A during thetesting of the circuit B, the pad 14 can be temporarily connected to thecircuit B during a testing procedure while the circuitry of the die 16is in a second or test mode of operation to allow test signals to beapplied to the circuit B. (For purposes of the invention, the details ofthe structure and operation of the circuits A and B are unimportant andtherefore these circuits are not discussed in detail.) The pad 14 cannotbe omitted from the die 16, however, because it is used in the first andother operating modes of the integrated circuit formed on the die 16. Aconfiguration circuit 18, described in detail below, is used to connectthe pad 14 to the circuit B during the test mode for the circuit B. Theoperation of the configuration circuit 18 is controlled by aconfiguration or map signal, a configure or map pad 20 to which the mapsignal is applied by external means. The map pad 20, which may besimilar in size and construction to the known test pads 10 (FIG. 2), isdisposed along one of the accessible sides 8.

In operation during testing of the die 16, the test apparatus drives hemap pad 20 with a configure or map signal. In response to this mapsignal, the configuration circuit 18 couples or maps the unused pad 14to the circuit B, which is normally driven by signals that are appliedto the used pad 12. That is, the circuit 18 configures the connectionsof the pads 12 and 14 so that the testing apparatus can drive thecircuit B via the accessible but unused pad 14. Therefore, by using onemap pad 20 and one or more unused pads 14, one can reduce the number ofor eliminate altogether the test pads 10 (FIG. 2), and thus reduce thearea of the die 16 as compared with that of known dies.

FIG. 4 is a block diagram of one embodiment of the configuration circuit18 of FIG. 3. The circuit 18 includes a first mapping or switchingcircuit 22 that has a first signal terminal coupled to a referencevoltage V_(REF), a second signal terminal coupled to the accessible butunused pad 14, a first control terminal coupled to the map pad 20, asecond control terminal coupled to an enable signal generator, here apad 21, that provides a signal {overscore (ENABLE)}, and a third signalterminal coupled to a conductive path or signal conductive path A. Thesignal conductive path A is also coupled to the conventional circuit A.The bar over {overscore (ENABLE)} indicates that it is active at a lowlogic level, i.e., logic 0. The circuit 18 also includes a secondmapping or switching circuit 24 that has a first signal terminal coupledto the pad 14, a second signal terminal coupled to the inaccessible butused pad 12, a first control terminal coupled to the map pad 20, asecond control terminal coupled to the enable pad 21, and a third signalterminal coupled to a conductive path or signal conductive path B. Thesignal conductive path B is also coupled to the conventional circuit B.

In operation, during the first or normal operational mode of theintegrated circuit on the die 16, the circuit 18 couples the pad 12 tothe circuit B and couples the pad 14 to the circuit A. The pad 20 isdriven with a first logic level to indicate this first mode ofoperation. Because many or all of the first and other nonwafer-testoperational modes are implemented after the die 16 has been packaged,and because one often lacks access to the pads 20 and 21 after the die16 is packaged, map signal and enable signal generator circuits (notshown) are often formed on the die 16 to drive the pad 20 with the mapsignal and the pad 21 with {overscore (ENABLE)}. An example of suchgenerators includes conventional pull-up or pull-down resistors orlatches. If such generators are used, the map pad 20 and the enable pad21 may be eliminated, and the map signal and enable signal generatorsmay be directly coupled to the appropriate internal nodes. When presentand driven externally, the map pad 20 and the enable pad 21 may beconsidered the map signal and enable signal generators, respectively.For example purposes, it is assumed that the map pad 20 and the enablepad 21 are present. In response to this first logic level and a logic 0for {overscore (ENABLE)}, the first mapping circuit 22 couples the pad14 to the circuit A via the signal conductive path A, and the secondmapping circuit 24 couples the pad 12 to the circuit B via the signalconductive path B.

In operation, the second or wafer-test procedure of the integratedcircuit is entered by the testing apparatus driving the pad 20 to asecond logic level, and driving the {overscore (ENABLE)} signal to alogic 0. The first mapping circuit 22 then decouples the pad 14 from theconductive path A, and drives the conductive path A with a fixed voltageV_(REF), which in one embodiment of the invention is a logic level. Alsoin response to the second logic level and {overscore (ENABLE)}, thesecond mapping circuit 24 couples the pad 14 to the circuit B via theconductive path B. The second mapping circuit 24 may also decouple thepad 12 from the conductive path B, and thus from the circuit B, althoughsuch decoupling is often unnecessary. Thus, in the test mode ofoperation, the testing apparatus can drive the circuit B, which isdriven by pad 12 in the first mode of operation, by driving the unusedbut accessible pad 14 whenever {overscore (ENABLE)} is logic 0.

FIG. 5 is a schematic diagram of one embodiment of the configurationcircuit 18 of FIG. 4. As shown, the first mapping circuit 22 includes aninverter 26 that has an output and an input coupled to the pad 20 forreceiving a signal MAP. A NAND gate 28 has a first input coupled to theoutput of the inverter 26, a second input, and an output coupled to theconductive path A. The NAND gate 28 also has supply terminals that arecoupled to V_(CC) (logic 1) and ground (logic 0). Depending upon thelogic level with which the first mapping circuit 22 drives conductivepath A during a test mode, either V_(CC) or ground is used as V_(REF)(FIG. 4). A buffer 29 has a signal input coupled to the pad 14, acontrol input coupled to {overscore (ENABLE)}, and an output coupled tothe second input of the NAND gate 28. Because only one buffer 29 buffersthe signal from the pad 14, {overscore (ENABLE)} is coupled to thebuffer 29 instead of the first mapping circuit 22 as shown in FIG. 4. Itis understood, however, that one can make various modifications to thecircuit of FIG. 5 such that {overscore (ENABLE)} is directly coupled tothe first mapping circuit 22 as shown in FIG. 4. For example, thecircuit 18 could include a first buffer in the first mapping circuit 22and a second buffer in the second mapping circuit 24, where both of thebuffers have signal inputs coupled to the pad 14 and control inputscoupled to {overscore (ENABLE)}.

The second mapping circuit 24 includes an inverter 30 that has an outputand an input coupled to {overscore (ENABLE)}. A NAND gate 32 has a firstinput coupled to the output of the inverter 30, has a second input, andhas an output. An inverter 34 has an input that is coupled to the pad20, and has an output coupled to the second input of the NAND gate 32.An inverter 36 has an input coupled to the pad 20 and has an output. Anelectronic switch 38 has a signal input coupled to the output of thebuffer 29, has first and second complementary control inputsrespectively coupled to the input and the output of the inverter 36, andhas an output coupled to the conductive path B. An inverter 40 has aninput that is coupled to the output of the inverter 34 and has anoutput. A buffer 42 has a signal input coupled to the pad 12, has acontrol input coupled to the output of the NAND gate 32, and has anoutput. An electronic switch 44 has a signal input coupled to the outputof the buffer 42, has first and second complementary control inputsrespectively coupled to the input and the output of the inverter 40, andhas an output coupled to the conductive path B.

In operation during the first or normal mode of operation, the circuit18 couples the pad 12 to the signal conductive path B and couples thepad 14 to the signal conductive path A. MAP is driven to an inactivelogic 0, typically by a circuit (not shown) formed on the die 16, asdiscussed above in conjunction with FIG. 4. The inverter 26 converts thelogic 0 MAP to a logic 1 {overscore (MAP)}, which enables the NAND gate28. The NAND gate 28 then acts as an inverter, and couples theconductive path A to the complement of the signal that drives the pad14. Although not shown, the circuit A of FIG. 4 may include an inverterto generate the original signal on pad 14 from its complement on theconductive path A. The NAND gate 32, in response to the logic 1{overscore (MAP)} signal at one of its inputs and the logic 1 at itsother input, generates at its output a logic 0 that enables the buffer42. Furthermore, the logic 1 {overscore (MAP)} signal and itscomplement, which the inverter 40 generates at its output, close theswitch 44 so that it couples the pad 12 to the conductive path B via thebuffer 42 whenever {overscore (ENABLE)} is at logic 0. Finally, thelogic 0 {overscore (MAP)}, both directly and through the inverter 36,disables the switch 38 to isolate the pad 14 from conductive path B.

In operation during the second or test mode, the testing apparatusdrives the pad 20, and thus MAP, to an active logic 1. The inverter 26provides a logic 0 {overscore (MAP)} to one of the inputs of the NANDgate 28, thereby disabling the NAND gate 28 so that conductive path A isdriven with a logic 1 independently of the signal driving the pad 14.Thus, the NAND gate 28 decouples the pad 14 from the conductive path A,and holds the conductive path A at a substantially constant voltageV_(REF), which in this embodiment is a logic 1 derived from V_(CC). Thelogic 1 MAP via the inverter 34 causes the NAND gate 32 to output alogic 1 which disables the buffer 42. The pad 12 is then decoupled fromany other portion of the circuit 18, including conductive path B tocircuit B. The logic 0 {overscore (MAP)}, both directly and via theinverter 40, opens the electronic switch 44. Thus, the switch 44 and thebuffer 42 decouple the pad 12 from the conductive path B. Furthermore,the logic 1 MAP, both directly and via the inverter 36, closes theswitch 38, which thus couples the pad 14 to the conductive path B viathe buffer 29. Thus, during the second or test mode, the unused butaccessible pad 14 is coupled to the conductive path B, and effectivelysubstitutes for the inaccessible pad 12 whenever {overscore (ENABLE)} isat logic 0.

It will be appreciated that, although specific embodiments of theinvention have been described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. For example, although discussed with respect toconfiguring pad connections for normal and test operational modes, theinvention may be used to configure the pads for two or more other typesof operational modes with or without the use of a plurality of mapsignals. Also, although the circuit of FIG. 5 uses the configurationcircuit 18 to configure the pads 12, 14 as inputs to the circuits A andB, it will also be understood that the pads 12, 14 may also beconfigured as outputs of the circuits A and B. Accordingly, theinvention is not limited except as by the appended claims.

We claim:
 1. A method for testing a first circuit in a semiconductorchip having a plurality of inaccessible pads disposed on a first sideinaccessible to a testing apparatus and further having a plurality ofaccessible pads disposed on a second side accessible to the testingapparatus, the first circuit coupled to receive a signal applied to aselected one of the plurality of inaccessible pads during a first mode,the method comprising: applying test signals generated by the testingapparatus to the plurality of accessible pads; providing the test signalapplied to a selected one of the plurality of accessible pads to thefirst circuit during a second mode; and decoupling the selected one ofthe plurality of inaccessible pads from the first circuit during thesecond mode.
 2. The method of claim 1 wherein providing the test signalcomprises electrically coupling the first circuit to the selected one ofthe plurality of accessible pads to receive the test signal appliedthereto.
 3. The method of claim 1 wherein decoupling comprises opening aswitch coupled between the selected one of the inaccessible pads and thefirst circuit.
 4. The method of claim 1, further comprising driving asecond circuit coupled to receive a signal from the selected one of theplurality of accessible pads during the first mode with a constantvoltage while the test signal applied to the selected one of theplurality of accessible pads is provided to the first circuit.
 5. Themethod of claim 4 wherein driving the second circuit comprises couplingthe second circuit to a constant voltage source.
 6. The method of claim4, further comprising decoupling the selected one of the accessible padsfrom the second circuit during the second mode.